Olmesartan medoxomil is chemically known as 4-(1-hydroxy-1-methylethyl)-2-propyl-1-[[2′-(1H-tetrazol-5-yl) [1,1′-biphenyl]-4-yl]methyl]-1H-imidazole-5-carboxylic acid (5-methyl-2-oxo-1,3-dioxol-4-yl)methyl ester and represented by formula I

Olmesaratan medoxomil (I) is a prodrug that is selective AT1 subtype angiotensin II receptor antagonist and pharmaceutically used as an antihypertensive for the treatment and prophylaxis of hypertension.
Olmesartan medoxomil (I) was first disclosed in U.S. Pat. No. 5,616,599, along with process for its preparation by treatment of trityl olmesartan medoxomil (III) with aqueous acetic acid. The filtrate obtained after removal of trityl alcohol was concentrated to give olmesartan medoxomil (I) containing about 4-5% of olmesartan acid impurity (II). This method is not industrially viable since, on a commercial scale the removal of large quantity of aqueous acetic acid would require longer durations causing higher rate of hydrolysis of olmesartan medoxomil (I) and producing excess of olmesartan acid impurity (II).

Another U.S. Pat. No. 7,528,258 provides process for preparation of olmesartan medoxomil (I) wherein trityl olmesartan medoxomil (III) is contacted with sulphuric acid in a water miscible organic solvent. The trityl alcohol was filtered off, base was added to the filtrate and olmesartan medoxomil (I) with about 1% of olmesartan acid impurity (II) was recovered. This method utilizes strong and corrosive acid like sulphuric acid and incorporate additional step of neutralizing the acid with a base.
Another application US 2008/0214637, describes process for preparation of olmesartan medoxomil (I), which involves reacting trityl olmesartan medoxomil (III) with catalytic amount of acid selected from trifluoroacetic acid, methanesulphonic acid etc. The process further involves pH adjustment of the reaction mixture utilizing a base followed by multiple extractions and concentration of solvents and thereafter recovery of product by crystallization/precipitation.
PCT application WO 2010/067913 describes process for preparation of olmesartan medoxomil (I) from trityl olmesartan (III), wherein the triphenylmethane group is removed by using an acidic cation exchange resin. This process involves resin which is pre-treated with acid and after the reaction the resin is filtered and the filtrate concentrated, the product is recovered by crystallization. This process involves additional steps like treating the resin with an acid, filtrations, recovery of the resin etc.
Patent applications US 2006/0074117 and US 2010/0076200 describes purification process to produce olmesartan medoxomil (I) containing less than 0.05% of olmesartan acid impurity (II), by crystallization from C3-6 ketone-water mixture. However, this process utilizes excess volumes of solvents.
Thus, there exists a need for improved process for preparation of olmesartan medoxomil (I) that can avoid use of strong acid, multiple steps, laborious work-up, large excess of solvent volumes etc.